Drill bit

ABSTRACT

A drill bit comprises a plurality of cutting blades arranged circumferentially, the cutting blades having a cutting edge formed by a connecting ridge line between a cutting face and a flank; and a tip formed by coupling radially inner ends of the cutting edges to each other at a center portion of a bit tip end, wherein at least one of the cutting blades has a first flank and a second flank, wherein the first flank has a blade tip side flank close to the cutting edge and a base end side flank formed on the base end side of the blade tip side flank, and wherein a tilt angle α 1  of the blade tip side flank  3   a  is smaller than a tilt angle α 2  of the base end side flank  3   b  which is tilted toward the axial direction.

TECHNICAL FIELD

The present invention relates to a drill bit, and particularly to adrill bit which is attached to a vibrating drilling device so as to beable to drill holes in stone, concrete, etc.

BACKGROUND ART

Conventionally, drilling holes in stone, concrete, etc. is done byattaching a drill bit made of cemented carbide or the like to avibrating drilling device and by applying both rotation and vibration(strike) in an axial direction to the drill bit.

One example of the drill bit of this type is constructed such that aplurality of cutting blades are arranged in a circumferential directionof the drill bit, a connecting ridge line between a cutting face and aflank which are formed in each cutting blade acts as a cutting edge, andradially inner ends of the cutting edges are coupled to each other at acenter portion of a bit tip end (see Patent Document 1).

Patent Document 1: Japanese Laid-Open Patent Application Publication No.2002-178328

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, in the drill bit constructed as above, since the flank locatedon the rear side of the cutting edge is a single plane having a constanttilt angle from a radially inner end portion to a radially outer end,the radially outer end that is a free end is easily chipped in the caseof causing the drill bit to strongly contact an object to be drilled,such as concrete, to carry out drilling operations. Meanwhile, in thecase of reducing the tilt angle of the flank which is tilted toward anaxial direction to prevent the radially outer end of a blade tip fromgetting chipped, an ability of crushing the object to be drilled, or thelike when strongly contacting it deteriorates. As a result, the drillbit deteriorates in drilling performance. In addition, as describedabove, when the flank is a face whose tilt angle is not large, the flankprojects toward the tip end side more than the cutting edge at aconnecting portion between the flank and the cutting face located on therear side of the flank in a rotational direction. Therefore, it isnecessary to form a contacting face by cutting this portion, and theconstruction of the drill bit becomes complex due to a negativetechnical reason.

The present invention was made in view of these circumstances, and anobject of the present invention is to provide a drill bit whose drillingperformance does not deteriorate and whose radially outer end of theblade tip is never chipped.

Means for Solving the Problems

The above object of the present invention is achieved by a drill bitdescribed below.

A drill bit according to the present invention comprises: a plurality ofcutting blades arranged in a circumferential direction thereof, thecutting blades being each structured to have a cutting edge formed by aconnecting ridge line between a cutting face and a flank thereof; and atip end portion formed by coupling radially inner ends of the cuttingedges to each other at a center portion of a bit tip end when viewedfrom bottom, wherein at least one of the cutting blades has a firstflank at an outer circumference end portion thereof and a second flankat an inner circumference side portion thereof, wherein the first flankhas a blade tip side flank close to the cutting edge and a base end sideflank formed on a base end side of the blade tip side flank, and whereina tilt angle of the blade tip side flank which is tilted toward an axialdirection is smaller than a tilt angle of the base end side flank whichis tilted toward the axial direction.

With the above construction of the drill bit, the radially outer endportion of the blade tip can be increased in rigidity, and therefore, itis possible to realize that the radially outer end portion is notsubstantially chipped. In addition, efficient drilling can be carriedout by causing the second flank tilted toward the axial direction andlocated on the radially inner side of the first flank to have a tiltangle, the tilt angle being set in view of a drilling efficiency (tiltangle set by emphasizing only the drilling efficiency (this tilt anglegenerally becomes larger than that on a radially outer side portion)).Then, since the tilt angle of the second flank can be set to be large,the connecting portion between the second flank and the cutting faceadjacently located on the rear side of the second flank in therotational direction can be set to be located more rearward toward thebase end side in the axial direction than the cutting edge. Therefore,it is possible to realize such a construction that it is unnecessary toform a conventional connecting face for preventing the projection towardthe tip end side.

Thus, in accordance with the drill bit of the present invention, it ispossible to achieve mutually contradictory technical objects that aresolving of chipping of the cutting blade and maintaining of highdrilling efficiency, and it is also possible to avoid “the increase incomplexity of the construction” caused due to the negative technicalreason.

Then, when, in the above drill bit, the tilt angle of the second flankwhich is tilted toward the axial direction is set to be larger than eachof the tilt angle of the blade tip side flank which is tilted toward theaxial direction and the tilt angle of the base end side flank which istilted toward the axial direction, it is possible to realize a preferreddrill bit whose drilling efficiency is high and whose radially outer endportion is not substantially chipped.

Moreover, when, in the above drill bit, first ends of the blade tips,which are located on the radially inner side, are coupled to each otherat a center of the bit tip end, it is possible to realize a preferreddrill bit which, when drilling, can easily carry out positioning of arotation center and can stably rotate about the rotation center.

Moreover, when, in the above drill bit, the cutting blade includes aplurality of main cutting blades and a plurality of sub cutting blades,and each of the main cutting blades includes the first flank and thesecond flank, the cutting and crushing of the sub cutting blades arecarried out in addition to the cutting and crushing of the main cuttingblades. Therefore, it is possible to realize a more efficient drill bit.

Moreover, when, in the above drill bit, the sub cutting blade isprovided between the main cutting blades adjacent to each other in arotational direction, and the sub cutting blade is provided at unequalintervals in the circumferential direction with respect to the maincutting blades located on both sides of the sub cutting blade, it ispossible to drill holes having better roundness in the case of drillingholes using vibration.

Moreover, when, in the above drill bit, the sub cutting blade isprovided so as to be located more rearward toward the base end side inthe axial direction than the main cutting blade, concrete, stone, or thelike crushed by the main cutting blade can be further crushed by the subcutting blade. Therefore, it is possible to realize the drill bit whosehole drilling efficiency is further high.

Moreover, when, in the above drill bit, the radially inner end of thecutting edge of the sub cutting blade is located so as to be spacedapart from a center of the bit tip end toward the outer circumferencewhen viewed from bottom, and a contacting face whose borders are definedby lines that are a ridge line formed by connecting the radially innerend of the cutting edge of the sub cutting blade and the center of thebit tip end when viewed from bottom, a concave line that is a connectingline between the contacting face and the cutting face of the maincutting blade adjacently located on a rear side of the contacting facein the rotational direction, and a line formed by connecting theradially inner end, that is one end of the ridge line, of the cuttingedge of the sub cutting blade and the radially outer end of the concaveline is formed at a portion which is located between two main cuttingblades adjacent to each other in the rotational direction and which islocated on a radially inner side portion of the sub cutting bladeprovided between the above two main cutting blades, the angle of thecutting face of the main cutting blade can be set to be larger than thatof the flank of the main cutting blade. Therefore, it is possible torealize the drill bit whose cutting and crushing abilities are high.

Moreover, when, in the above drill bit, a material thereof is cementedcarbide, it is possible to realize the drill bit whose drillingperformance is preferable and whose durability is excellent.

EFFECTS OF THE INVENTION

In accordance with the drill bit of the present invention, it ispossible to provide the drill bit whose drilling performance does notdeteriorate and whose radially outer end of the blade tip is neverchipped.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a bottom view showing a construction of a main part of a drillbit according to one embodiment of the present invention.

FIG. 2 is a side view of the above construction when viewed from adirection indicated by arrows II-II shown in FIG. 1.

FIG. 3 is a side view of the above construction when viewed from adirection indicated by arrows III-III shown in FIG. 1.

FIG. 4 is a side view of the above construction when viewed from adirection indicated by arrows IV-IV shown in FIG. 1.

FIG. 5 is a side view of the above construction when viewed from adirection indicated by arrows V-V shown in FIG. 1.

FIG. 6 is a perspective view showing a construction of a main part ofthe drill bit shown in FIGS. 1 to 5.

FIG. 7 are views showing the tilt angles of respective flanks of thedrill bit shown in FIGS. 1 to 6. FIG. 7( a) is the same as FIG. 2 and isthe side view of the above construction when viewed from a side of aflank orthogonal to a connecting ridge line, FIG. 7( b) is a partiallyenlarged view showing tilt angles α1 and α2 of the flank when a crosssection taken along line VIIb-VIIb shown in FIG. 7( a) is viewed from adirection indicated by an arrow Z (connecting ridge line direction), andFIG. 7( c) is a partially enlarged view showing a tilt angle α3 of theflank when a cross section taken along line VIIc-VIIc shown in FIG. 7(a) is viewed from the direction indicated by the arrow Z.

FIG. 8 is a perspective view showing a construction of a main part of adrill bit according to another embodiment (Embodiment 2).

FIG. 9 is a bottom view showing the construction of the main part of thedrill bit shown in FIG. 8.

FIG. 10 is a side view of the above construction when viewed from adirection indicated by arrows X-X shown in FIG. 9.

FIG. 11 is a side view of the above construction when viewed from adirection indicated by arrows XI-XI shown in FIG. 9.

FIG. 12 is a side view of the above construction when viewed from adirection indicated by arrows XII-XII shown in FIG. 9.

FIG. 13 is a side view of the above construction when viewed from adirection indicated by arrows XIII-XIII shown in FIG. 9.

FIG. 14 are views showing the tilt angles of respective flanks of thedrill bit shown in FIGS. 8 to 13. FIG. 14( a) is the same as FIG. 10 andis the side view of the above construction when viewed from a side of aflank orthogonal to a connecting ridge line, FIG. 14( b) is a partiallyenlarged view showing tilt angles α1 and α2 of the flank when a crosssection taken along line IXVb-IXVb shown in FIG. 14( a) is viewed from adirection indicated by an arrow Z (connecting ridge line direction), andFIG. 14( c) is a partially enlarged view showing a tilt angle α3 of theflank when a cross section taken along line IXVc-IXVc shown in FIG. 14(a) is viewed from the direction indicated by the arrow Z.

EXPLANATION OF REFERENCE NUMBERS

A: drill bit

1: cutting blade

2: cutting face

3: flank

4: ridge line

10: tip end portion

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the drawings.

Embodiment 1

Hereinafter, embodiments of a drill bit according to the presentinvention will be specifically described with reference to the drawings.

As shown in FIGS. 1 and 6, a tip end portion 10 of a drill bit Aaccording to Embodiment 1 includes a plurality of cutting blades 1(three cutting blades 1 in the present embodiment) arranged in acircumferential direction to extend substantially radially. Each cuttingblade 1 includes a cutting face 2 formed on a front side in a rotationaldirection R when carrying out a drilling operation and a flank 3adjacently formed on the rear side of the cutting face 2 in therotational direction. The cutting blade 1 also includes a connectingridge line 4 that is a ridge line formed by a connection between thecutting face 2 and the flank 3. Respective positions of the connectingridge line 4 in a radial direction of the drill bit A are located at atip end in an axial direction. Thus, the connecting ridge line 4constitutes a cutting edge of the drill bit A.

First ends of the three connecting ridge lines 4, which are located on aradially inner side, are coupled to each other at a center O1 of thedrill bit A when viewed from bottom, and the center O1 is the rotationcenter when carrying out the drilling operation. However, according toneed, those first ends of the three connecting ridge lines 4 may becoupled to each other at a center portion of a bottom face of the drillbit A via a so-called “chisel edge”.

As shown in FIGS. 1 to 6, the flank 3 of each of the three cuttingblades 1 includes a first flank 3A at an outer circumference end portion(radially outer end portion) thereof and a second flank 3B on an innercircumference side (radially inner side) thereof. The first flank 3A andthe second flank 3B are coupled to each other in the radial directionvia a connection face 3C that is a curved face. Note that the connectionface 3C does not have to be the curved face.

The first flank 3A includes a blade tip side flank 3 a close to thecutting edge (connecting ridge line 4) and a base end side flank 3 bformed on a base end side of the blade tip side flank 3 a.

As shown in FIG. 7, the drill bit A is constructed such that a tiltangle α1 of the blade tip side flank 3 a which is tilted toward theaxial direction is smaller than a tilt angle α2 of the base end sideflank 3 b which is tilted toward the axial direction.

Moreover, in the present embodiment, a tilt angle α3 (see FIG. 7) of thesecond flank 3B which is tilted toward the axial direction is largerthan each of the tilt angle α3 of the blade tip side flank 3 a which istilted toward the axial direction and the tilt angle α2 of the base endside flank 3 b which is tilted toward the axial direction. However, thepresent embodiment is not limited to the above constructions, as long asthe tilt angle α3 of the second flank 3B which is tilted toward theaxial direction is larger than at least the tilt angle α1 of the bladetip side flank 3 a which is tilted toward the axial direction.Therefore, the tilt angle α3 of the second flank 3B which is tiltedtoward the axial direction may be equal to or smaller than the tiltangle α2 of the base end side flank 3 b which is tilted toward the axialdirection.

Specifically, in the case of the drill bit for use in concrete (stone,etc.), the tilt angle α1 is set to approximately 23 degrees, the tiltangle α2 is set to approximately 45 degrees, and the tilt angle α3 isset to approximately 50 degrees. However, these tilt angles are setsuitably depending on the use, use condition, material, etc. of thedrill bit A.

Moreover, as shown in FIGS. 1 and 6, and FIGS. 2 to 5, connectingconcave lines 5 corresponding to the connecting ridge lines 4 extendradially from the rotation center O1, the connecting concave lines 5each being formed by the second flank 3B of one cutting blade 1 locatedon the front side in the rotational direction R and the cutting face 2of another cutting blade 1 adjacently located on the rear side of thatone cutting blade 1. When viewed from bottom, the connecting concavelines 5 and the connecting ridge lines 4 are disposed alternately in therotational direction and extend radially.

Further, in order that a rear wall face 6 located on the rear side ofthe first flank 3A, the connection face 3C and the second flank 3B inthe rotational direction is formed as a solid face in the radialdirection at a connection portion formed at the base end of the tip endportion 10 of drill bit A, a rear adjustment face 7 is formed at aportion located on the base end side of from the second flank 3B to theconnection face 3C.

Similarly, a front adjustment face 9 is formed which connects thecutting face 2, having an arbitrary angle, of the cutting blade 1 to afront wall face 8 which extends toward the base end of the drill bit Aand is located on the front side in the rotational direction.

In the present embodiment, the tip end portion 10 of the drill bit A isformed of an integral block body, and its material is cemented carbide.Note that the tip end portion 10 of the drill bit A can, of course, alsobe made of a material other than cemented carbide.

Moreover, a shaft portion (not shown) of the drill bit is integrallyfixed on the base end side (lower side in FIGS. 2 to 5) of the tip endportion 10 of the drill bit A, the shaft portion being provided with adischarge groove formed to extend circumferentially and an attachingportion (shank portion) to be attached to a drilling device.

The drill bit A according to the present embodiment constructed as abovecan provide the following operational advantages in the drillingoperation.

To be specific, as described above, since, in the drill bit A, the tiltangle α1 of the blade tip side flank 3 a which is tilted toward theaxial direction is smaller than the tilt angle α2 of the base end sideflank 3 b which is tilted toward the axial direction, that is, since thetilt angle α1 of the blade tip side flank 3 a which is tilted toward theaxial direction is more obtuse than the tilt angle α2 of the base endside flank 3 b which is tilted toward the axial direction, the outer endportion of the cutting edge is not substantially chipped even when animpact force is applied thereto in the drilling operation.

Moreover, since the tilt angle α3 of the second flank 3B and the tiltangle α2 of the base end side flank 3 b of the first flank 3A are large,the present drill bit A can deliver a high drilling performance eventhough the cutting edge is configured not to be substantially chipped.

Further, since the tilt angle α3 of the second flank 3B can be set to belarge, the connecting concave line 5 formed between the second flank 3Band the cutting face 2 located on the rear side of the second flank 3Bin the rotational direction R can be realized as a single, simple line,and in addition, it is unnecessary to form a contacting face that hasbeen necessary in a conventional drill bit due to a negative reason. Onthis account, it is possible to simplify the tip end portion 10 incontrast to conventional ones.

In addition, although the tip end portion 10 of the drill bit A of thepresent embodiment is constructed as above by a complicated combinationof a comparatively large number of faces, the tip end portion 10 isformed of an integral block body, so that it can be manufactured withhigh production efficiency by casting, forging, or the like.

Embodiment 2

Next, another embodiment (Embodiment 2) of the present invention will bedescribed with reference to FIGS. 8 to 14. In these drawings, areference number obtained by adding 100 to the reference number used inEmbodiment 1 is used for a component identical with or corresponding tothat in Embodiment 1.

A tip end portion 110 of a drill bit A2 according to Embodiment 2 isconstructed as follows.

In Embodiment 2, a plurality of main cutting blades 100A (three maincutting blades 101A in the present embodiment) are arranged in therotational direction R to extend substantially radially, and each of subcutting blades 101B is formed between adjacent main cutting blades 101A.In the present embodiment, the cutting blades are the main cuttingblades 101A and the sub cutting blades 101B.

The sub cutting blade 101B is provided so as to be located more rearwardthan the main cutting blade 101A in a direction from the tip end (thecenter O1 in the present embodiment) toward the base end side.Therefore, the cutting edge (connecting ridge line 124) of the subcutting blade 101B is located more rearward toward the base end sidethan the cutting edge (connecting ridge line 104) of the main cuttingblade 101A in the axial direction.

In the present embodiment, a plurality of main cutting blades 101A(three main cutting blades 101A) are provided at unequal intervals inthe circumferential direction. In contrast, the sub cutting blades 101Bare provided at equal intervals (interval of 120 degrees) in thecircumferential direction.

Moreover, the sub cutting blade 101B is provided at unequal intervalswith respect to the main cutting blades 101A adjacently located on thefront side and rear side of this sub cutting blade 101B in therotational direction R. That is, each sub cutting blade 101B is providedso that an interval (arrangement angle) between the sub cutting blade101B and the main cutting blade 101A adjacently located on the frontside of this sub cutting blade 101B in the rotational direction R and aninterval (arrangement angle) between the sub cutting blade 101B and themain cutting blade 101A adjacently located on the rear side of this subcutting blade 101B in the rotational direction R are different from eachother. However, although not shown, instead of the present embodiment,the main cutting blades 101A may be provided at equal intervals(interval of 120 degrees) in the circumferential direction, the subcutting blades 101B may be provided at unequal intervals in thecircumferential direction, and the sub cutting blade 101B may beprovided at unequal intervals with respect to the main cutting blades101A adjacently located on the front side and rear side of this subcutting blade 101B in the rotational direction R. Alternatively, themain cutting blades 101A may be provided at equal intervals (interval of120 degrees) in the circumferential direction, the sub cutting blades101B may be provided at equal intervals (interval of 120 degrees) in thecircumferential direction, and the sub cutting blade 101B may beprovided at unequal intervals with respect to the main cutting blades101A adjacently located on the front side and rear side of this subcutting blade 101A in the rotational direction R.

A radially inner end 124 e of the cutting edge (connecting ridge line124) of the sub cutting blade 101B is located so as to be spaced apartfrom the center O1 of the bit tip end toward the outer circumference(radially outer side) when viewed from bottom.

A contacting face 120 is formed at a portion which is located betweenthe cutting face 102 of one main cutting blade 101A and the second flank103B of another main cutting blade 101A located on the front side ofthat one main cutting blade 101A in the rotational direction R and whichis located on a radially inner side of the cutting edge (connectingridge line 124) of the sub cutting blade 101B. In other words, theborders of the contacting face 120 are defined by a ridge line 120 aformed by connecting the radially inner end of the cutting edge(connecting ridge line 124) of the sub cutting blade 101B and the centerO1 of the bit tip end when viewed from bottom, a concave line that is aconnecting line 120 b between the contacting face 120 and the cuttingface 102 of the main cutting blade 101A adjacently located on the rearside of the contacting face 120 in the rotational direction, and a line120 c formed by connecting the radially inner end (one end of the ridgeline 120 a) of the cutting edge (connecting ridge line 124) of the subcutting blade 101B and the radially outer end of the concave line(connecting line 120 b).

As above, in the present embodiment, in order to improve the cuttingefficiency, a clearance angle of the second flank 103B of the maincutting blade 101A is set to be more acute-angled than a rake angle ofthe cutting face 102.

Moreover, as clearly shown in FIGS. 12 and 13, the cutting edge(connecting ridge line 124) of the sub cutting blade 101B bends in aposition thereof in the radial direction on a radially outer side towardthe base end side, and the tilt angle of a radially outer side portion124A of the cutting edge (connecting ridge line 124) toward the base endside is more acute-angled. In other words, the tilt angle of a radiallyinner side portion 124B of the cutting edge (connecting ridge line 124)is obtuse, and specifically, the radially inner side portion 124B isalmost horizontal to obtain a high crushing ability.

Moreover, the main cutting blade 101A basically has the sameconstruction as the cutting blade 1 (see FIG. 1) of Embodiment 1. Thatis, the main cutting blade 101A includes a cutting face 102 formed on afront side in the rotational direction R when carrying out the drillingoperation and a flank 103 adjacently formed on the rear side of thecutting face 102 in the rotational direction. The main cutting blade101A also includes a connecting ridge line 104 that is a ridge lineformed by a connection between the cutting face 102 and the flank 103.Respective positions of the connecting ridge line 104 in the radialdirection of the drill bit A2 are located at a tip end in the axialdirection. Thus, the connecting ridge line 104 constitutes the cuttingedge of the main cutting blade 101A of the drill bit A2.

First ends of the three connecting ridge lines 104, which are located onthe radially inner side, are coupled to each other at the center O1 ofthe drill bit A when viewed from the bottom, and the center O1 is therotation center when carrying out the drilling operation.

As shown in FIGS. 8 to 14, the flank 103 of each of the three maincutting blades 101A includes a first flank 103A at an outercircumference end portion (radially outer end portion) thereof and asecond flank 103B on an inner circumference side (radially inner side)thereof. The first flank 103A and the second flank 103B are coupled toeach other in the radial direction via a connection face 103C that is acurved face. Note that the connection face 103C does not have to be thecurved face.

The first flank 103A includes a blade tip side flank 103 a close to thecutting edge (connecting ridge line 104) of the main cutting blade 101Aand a base end side flank 103 b formed on the base end side of the bladetip side flank 103 a.

As with the cutting blade 1 shown in FIG. 7, the main cutting blade 101Aof the drill bit A2 is constructed such that as shown in FIG. 14, thetilt angle α1 of the blade tip side flank 103 a which is tilted towardthe axial direction is smaller than the tilt angle α2 of the base endside flank 103 b which is tilted toward the axial direction.

Moreover, in the present embodiment, the tilt angle α3 (see FIG. 14) ofthe second flank 103B which is tilted toward the axial direction islarger than each of the tilt angle α1 of the blade tip side flank 103 awhich is tilted toward the axial direction and the tilt angle α2 of thebase end side flank 103 b which is tilted toward the axial direction.However, the present invention is not limited to the aboveconstructions, as long as the tilt angle α3 of the second flank 103Bwhich is tilted toward the axial direction is larger than at least thetilt angle α1 of the blade tip side flank 103 a which is tilted towardthe axial direction. Therefore, the tilt angle α3 of the second flank103B which is tilted toward the axial direction may be equal to orsmaller than the tilt angle α2 of the base end side flank 103 b which istilted toward the axial direction.

Specifically, the tilt angles are preferably the same as those inEmbodiment 1. However, these tilt angles are set suitably depending onthe use, use condition, material, etc. of the drill bit A.

Also in the present embodiment, the tip end portion 110 of the drill bitA2 is formed of an integral block body, and its material is cementedcarbide. Note that the tip end portion 110 of the drill bit A2 can, ofcourse, also be made of a material other than cemented carbide.

Moreover, a shaft portion (not shown) of the drill bit is integrallyfixed on the base end side (lower side in FIGS. 8 and 10) of the tip endportion 110 of the drill bit A2, the shaft portion being provided with adischarge groove formed to extend circumferentially and an attachingportion (shank portion) to be attached to a drilling device.

In the drilling operation, the drill bit A2 according to the presentembodiment constructed as above can provide the same operationaladvantages as that according to Embodiment 1 by the same components asthose in Embodiment 1, and can further provide the following operationaladvantages by the specific components in Embodiment 2. That is, sincethe rake angle of the cutting face 102 is set to be more acute-angledthan the clearance angle of the second flank 103B of the main cuttingblade 101A by providing the contacting face 120 at the above-describedposition, a so-called biting performance of the drill bit is excellent,and high cutting efficiency can be obtained. In addition, since the subcutting blade 101B is provided as above, an object to be drilled crushedby the main cutting blade 101A can be effectively crushed, and thecrushed pieces can be discharged from the above-described dischargegroove to outside the drilled hole.

INDUSTRIAL APPLICABILITY

The present invention is applicable to a drill bit which is attached toa vibrating drilling device so as to be able to drill holes in stone,cement, etc.

1. A drill bit comprising: a plurality of cutting blades arranged in a circumferential direction thereof, the cutting blades being each structured to have a cutting edge formed by a connecting ridge line between a cutting face and a flank thereof; and a tip end portion formed by coupling radially inner ends of the cutting edges to each other at a center portion of a bit tip end when viewed from bottom, wherein at least one of the cutting blades has a first flank at an outer circumference end portion thereof and a second flank at an inner circumference side portion thereof, wherein the first flank has a blade tip side flank close to the cutting edge and a base end side flank formed on a base end side of the blade tip side flank, wherein a tilt angle of the blade tip side flank which is tilted toward an axial direction is smaller than a tilt angle of the base end side flank which is tilted toward the axial direction, and wherein a tilt angle of the second flank which is tilted toward the axial direction is larger than each of the tilt angle of the blade tip side flank which is tilted toward the axial direction and the tilt angle of the base end side flank which is tilted toward the axial direction.
 2. The drill bit according to claim 1, wherein the cutting edges include first ends, and wherein the first ends are coupled to each other at a center of the bit tip end when viewed from bottom, and further wherein the first ends are located on a radially inner side.
 3. The drill bit according to claim 1, wherein the drill bit comprises a cemented carbide material.
 4. A drill bit comprising: a plurality of cutting blades arranged in a circumferential direction thereof, the cutting blades being each structured to have a cutting edge formed by a connecting ridge line between a cutting face and a flank thereof; and a tip end portion formed by coupling radially inner ends of the cutting edges to each other at a center portion of a bit tip end when viewed from a bottom, wherein at least one of the cutting blades has a first flank at an outer circumference end portion thereof and a second flank at an inner circumference side portion thereof, wherein the first flank has a blade tip side flank close to the cutting edge and a base end side flank formed on a base end side of the blade tip side flank, wherein a tilt angle of the blade tip side flank which is tilted toward an axial direction is smaller than a tilt angle of the base end side flank which is tilted toward the axial direction, and wherein the cutting blades include a plurality of main cutting blades and a plurality of sub cutting blades, and each of the main cutting blades includes the first flank and the second flank.
 5. The drill bit according to claim 4, wherein each of the plurality of sub cutting blades is provided between the main cutting blades adjacent to each other in a rotational direction, and each of the plurality of sub cutting blades is provided at unequal intervals in the circumferential direction with respect to the main cutting blades located on both sides of each of the plurality of sub cutting blades.
 6. The drill bit according to claim 5, wherein a radially inner end of the cutting edge of each of the plurality of sub cutting blades is located so as to be spaced apart from a center of the bit tip end toward the outer circumference when viewed from bottom, and wherein a contacting face whose borders are defined by lines that are a ridge line formed by connecting the radially inner end of the cutting edge of each of the plurality of sub cutting blades and the center of the bit tip end when viewed from bottom, a concave line that is a connecting line between the contacting face and the cutting face of each of the plurality of main cutting blades adjacently located on a rear side of the contacting face in the rotational direction, and a line formed by connecting the radially inner end of the ridge line of the cutting edge of each of the plurality of sub cutting blades and a radially outer end of the concave line, the radially outer end of the concave line formed at a portion which is located between two main cutting blades adjacent to each other in the rotational direction and which is located on a radially inner side portion of an intervening one of the plurality of sub cutting blades provided between said two main cutting blades.
 7. The drill bit according to claim 4, wherein the cutting edge of each of the plurality of sub cutting blades is provided so as to be located more rearward toward the base end side in the axial direction than the cutting edge of each of the plurality of main cutting blades. 